The collimation or overlaying of X-rays should generally be understood to be the adaptation of the X-ray bundle emitted by X-ray source of the X-ray system to the scan field of view of the X-ray detector used. In this context, the term is not restricted to one of the system axes of the X-ray system. Hence, collimation can be implemented in the feed direction of the X-ray device, if provided, or along a system direction perpendicular thereto, for example in the fan direction of a computed tomography system or in any direction. In principle, collimation is used to reduce the X-ray dose for the examination object and is hence an essential operating instruction for all medical imaging procedures in the sense of the ALARA principle. In addition, suitable collimation generally enables X-ray detector overbeaming to be achieved and generally stray radiation to be suppressed.
Nowadays, the overlaying of X-rays in X-ray systems for medical imaging, such as, for example, computed tomography systems or C-arm X-ray devices is in principle performed in two different ways. On the one hand, fixed diaphragms with unchangeable diaphragm apertures are used, which typically do not allow the passage of at least an outer sub-region of an X-ray bundle depending on size and position relative to the X-ray, by way of absorption in the diaphragm material. Only the part of the X-rays allowed to pass reaches the examination object in the further beam path and is used for imaging. Fixed diaphragms save costs and space since a plurality of diaphragm apertures with different sizes can be arranged on a diaphragm in the positioning direction and, depending upon the application, can be moved into or out of the X-ray via a single drive. However, they have the drawback with respect to the size of diaphragm apertures that only a few inflexible variants are available for selection and these are not optimally suitable for each examination using an X-ray device.
On the other hand, use is made of diaphragm blades that can be moved or positioned separately with respect to one another and which enable the individual, and in particular dynamic, adjustment of the size of a diaphragm aperture. With diaphragms of this kind, it is even possible to vary the size of the diaphragm dynamically during an X-ray examination in order advantageously to keep the dose to which a patient is exposed as low as possible. Obviously, this requires positioning mechanisms with sufficient speed and precision, which, as a rule, entails increased production costs for the X-ray system. In addition, more installation space is required.
In addition, in order to reduce the total dose applied or stray radiation and to avoid detector overbeaming, it is also desirable to adapt the intensity profile or the energy spectrum of the X-rays to the special circumstances of an X-ray examination within the X-ray bundle. To this end, it is possible to use filters in the form of intensity filters (depending on their shape, so-called wedge or bowtie filters) or spectral filters (for example an Sn filter). The filters are characterized in that, due to the generally partial absorption of the X-rays, they attenuate the intensity of the X-rays or only absorb X-rays with a specific energy or do both simultaneously. To adapt the intensity profile, the filter use has to be adapted exactly to the absorption profile of the examination object in order to the keep the dose to which the examination object is exposed as low as possible.
If required, in modern X-ray systems, these filters can also be introduced into the X-ray beam.